Technical Abstract:
The sorption of chemical species onto soils is commonly quantified through the use of a distribution coefficient (Kd). This factor relates the chemical absorbed to the soil minerals per unit mass to the amount of chemical remaining in the solution in contact with the soil at equilibrium. The Kd values have been used extensively in the modeling of environmental transport of chemicals. There have been numerous laboratory procedures developed to measure the distribution coefficient; however, a majority of these methods rely on saturated (or solution conditions) that would not be realistic for vadose zone investigations. This is particularly important from soil fumigation studies. Incorrect distribution coefficients can lead to erroneous results of predictive transport modeling. Static and multiple extraction headspace methods have been modified to aid in the determination of the Kd factor for chloropicrin and methyl isothiocyanate at multiple levels of water saturation. The results of the preliminary investigations into this methodology and comparison to the traditional batch studies will be presented. The dependency of the Kd factor on soil water content will improve the mathematical modeling of the fate of soil fumigants.